Annealing of PA46 expands gear design by enhancing strength

Paul Boughton

Post-moulding annealing provides significant enhancement of the properties of products made from polyamide 46 (PA46; one PA46 formulation, Stanyl is marketed by DSM Engineering Plastics). Annealing results in reduced water uptake and enhanced mechanical properties, including 50 per cent greater stiffness and strength, increased molecular weight and greater resistance to fatigue.

A useful body of engineering knowledge about the annealing of PA46 has been developed, primarily to support the growing use of PA46 in gears, which are subjected to various different forces, including tangential, radial and (particularly in helical designs) axial.

Thermoplastics have supplanted metal for gears in a variety of applications.  Engineering thermoplastics offer electrical, mechanical and chemical advantages; can require little or no lubrication; weigh less; and can be moulded in more geometries, more quickly than metal ones.

For gears, polyamide (PA) resins – traditionally PA 6 and 6,6 (PA6/66) – have been popular for decades. Unfortunately, PA6/66 fails under significant ambient or torque/RPM-generated heat.  The crystallinity and glass transition temperature (Tg) of PA46 confer greater stiffness and strength at elevated temperatures than PA6/66, making it a material of choice in these conditions.  Annealing after moulding further enhances these engineering properties.

Structure of PA46

PA46 (melt T, 295°C; Tg, 80°C) consists of symmetric chains of adipic acid and diaminobutane (see Fig. 1).

In PA46, every amide bond is accompanied by 4 CH2 moieties. These regular chains provide rapid crystallisation at around a 70 per cent level (compare PA66 and PA6, at 50 per cent – and polyphthalamide (PPA), at 30 per cent.) Rapid crystallisation of PA46 not only reduces production cycle times, it results in a fine spherulitic structure with relatively high impact values – about 10 KJ/m2 compared with 5–7 KJ/m2 for PA6/66, Dry As Moulded (DAM). More importantly, PA46 retains strength and stiffness at temperatures above Tg.

Annealing creates positive changes in PA46. Annealing is a high temperature treatment of the material above the material’s Tg, but below its melting temperature. Annealing's effects are irreversible, and because annealing is accompanied by solid state postcondensation, molecular weight increases.

Polyamides absorb moisture to a level dictated by material polarity (hydrophilic tendency) and crystallinity. PA46 is a high polarity material; further, the high crystallinity of PA46 should reduce water uptake, as absorption takes place only in the material’s amorphous phase. Despite these two positives, however, PA46 shows high uptake levels (Table 1).

Table 1. Water uptake of Stanyl, PA66 and PA6 at various per centage relative humidity (RH)

            50 per cent RH    75 per cent RH    100 per cent RH

PA6            3.5            5.5            11
PA66            3            5            9
PA46            4            7            13
Annealed PA46    2            3.5            6.5
PPA            2.5            4.2            6

The higher moisture absorption of Stanyl is caused by relatively low density in the amorphous phase. Cooling triggers both crystal formation and amorphous-phase chain conformations with relatively high free volume, open to water absorption. Annealing PA46 substantially decreases the material’s water uptake (Fig. 3) by creating a dense amorphous phase.

Lower water uptake in annealed PA46 naturally results in fewer water-related dimensional and mechanical changes (see Table 2).

Table 2. Per Cent Dimensional Changes With and Without Annealing, Exposure to water at 100 per cent relative humidity

                        Change without annealing    Change with annealing*

PA46 pitch diameter change (per cent)    2.8                1.2
PA46 thickness change (per cent)**    4.2                2.1
PA6 pitch diameter change (per cent)    1.2                NA
PA6 thickness change (per cent)        2.0                NA
* 24 hours at 230°C  
** A steel hub increased expansion in the radial direction.

Importantly, after annealing, dimensional changes of parts operating at elevated temperatures follow the predictable and consistent coefficient of linear thermal expansion (CLTE), rather than moisture uptake.

In addition to reducing water uptake, annealing enhances the mechanical properties of PA46 above Tg. Fig. 4 shows dynamic mechanical analysis (DMA) curves comparing unfilled PA46 (DAM and annealed) with PA66 and POM.

Annealing PA46 increases modulus up to 50 per cent at temperatures above Tg. Tensile strength is similarly increased (Fig. 2. stress at 2 per cent strain).

In addition, annealed PA46 exhibits better fatigue performance at 140¢ªC than untreated material.

Initial studies indicate that annealing enhances wear and friction (W&F) performance of PA46 as well. A polytetrafluoroethylene (PTFE)-modified PA46 for dry-running applications was studied according to ASTM D-3702 for a torsion dampener application (Table 3).

Table 3. Wear and friction of torsion dampener made of PA46.

                PA46*        PA46* annealed (4 hrs 240¢ªC)

Wear rate ºm/hr        5.3        2.5
Coefficient of friction#    0.2.        0.15
* Modified with polytetrafluoroethylene (PTFE).
# Against steel.
Note that after annealing, wear rate is more than halved and the friction coefficient is substantially reduced.

Designers replaced a larger, slower motor with a smaller, faster one. PA66 failed when higher rpm generated frictional heat above its 260°C melting point. Unreinforced, annealed PA46 (melting point 295°C) solved this problem, retaining strength at temperatures above 26 °C. The new design saved costs and reduced space requirements by about 20 per cent.

Starter motor ring gears perform under harsh operating conditions and are required to survive 50 000 starts at 130°C, pass stall tests and operate at -30°C. DSM’s PA46 was the only material that fulfilled all the tests at leading starter maker Delco Remy.

Electric automotive power steering systems (Fig. 3), currently replacing hydraulic systems, gain from PA46 gears, which prevent noise transmission from steering shaft and wheels to the car interior. Norms are stringent – breakage of a tooth can cause steering system failure. The gear must operate at 120–130°C for the lifetime of the car (4000 hrs). Unreinforced grades of PA46 are used.

PA46 excels in applications that require strength at elevated temperatures. Annealing further enhances the attributes of PA46 by reducing water uptake, increasing strength and increasing options open for designers for smaller, faster mechanical components. Extensive study has resulted in a referenceable body of work detailing annealing process parameters and their effects on PA46, supporting the engineering of components for demanding applications.

Dr H K van Dijk is with DSM Engineering Plastics; Dr Ir H G H van Melick is with DSM Research; J Koenen is with DSM Engineering Plastics, Geleen, The Netherlands.


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